[CF-metadata] What do models assume for the shape of the Earth?

[John Caron]

Im finally getting back to investigating how to connect netCDF to GIS 
systems.

Here's a question to anyone who knows how models typically work:

Normally I dont see information in the netCDF file on the assumed shape 
of the Earth, eg ellipsoidal flattening or even spherical radius. Is 
that because it doesnt matter that much? Or is there an assumed standard 
that everyone just knows about?

Is there a different answer for global vs regional models?  At what 
scale does this start to matter?

thanks for any info